1. Field of the Invention
This application relates to hearing aids. More specifically, it relates to hearing aids comprising wireless receivers. Still more specifically the invention relates to hearing aids comprising frequency-shift-keying (FSK) receivers.
2. Prior Art
A common signal source in a hearing aid is one or more microphones picking up acoustic sound signals occurring in the vicinity of the hearing aid. Another common signal source in hearing aids is a telecoil receiver. Such a receiver is usually embodied as a tiny coil configured to pick up electromagnetic base band (i.e. unmodulated) audio frequency signals from a telecoil transmitter surrounding the hearing aid comprising the receiver.
State-of-the-art hearing aids are usually designed to accept more than one signal source for advanced functionalities for the purpose of amplifying, conditioning and reproducing them by virtue of the hearing aid circuitry.
Some behind-the-ear (BTE) hearing aids have means for connecting external equipment to the hearing aid circuitry, such as FM-receivers, Bluetooth® receivers, cables etc. Such external equipment enables communication with the hearing aid in various ways. Thus e.g. a cable connection may be provided for the purpose of programming the hearing aid, an FM-receiver may be connected for use in public address situations where a speaker is wearing a microphone with a wireless FM transmitter, and a Bluetooth® receiver may be used for streaming audio signals from a mobile telephone or the like.
Some newer hearing aid types also comprise internal wireless receivers. Most of these wireless receiver types draw their power directly from the hearing aid battery. Prolonged use of wireless receivers known in the art may lead to rapid depletion of the hearing aid battery necessitating frequent battery changes and adding to the cost of operation of the hearing aid. Receiver types having integral power supplies comprising a separate battery add to the weight, size and complexity of the receiver. A more power-efficient wireless receiver would thus be of great benefit to hearing aid users.
Power-efficiency may, e.g., be enhanced by reducing the total power consumption of the receiver circuitry. However, this should be performed without impairing the noise performance of the receiver, which would lead to reduced signal quality. Provided that signals to be transmitted are in a digital format, an FSK transmitter-receiver configuration, well-known to persons skilled in the art, is generally preferred.
FSK signals may be demodulated in several different ways, each having different advantages, topologies and complexity. The demodulators can be subdivided into several categories: FM to AM demodulator types (e.g. Slope, Foster-Seeley and Ratio), PLL demodulators, Zero-crossing demodulators and Quadrature demodulators.
One quadrature demodulator type well known in the art comprises a local oscillator and two signal branches denoted the in-phase branch and the quadrature branch, respectively, the received signal being splitted into an in-phase (I) component and a quadrature (Q) component. In the (binary) quadrature signal, one component is assigned binary zero, and the other component is assigned binary one. As the two signal components I and Q are mutually exclusive, a digital bitstream consisting of ones and zeroes is generated whenever the transmitter is active. Both branches are connected to a CPU, which completes the demodulation process. Generally, each branch comprises a multiplier, a filter and a decision device. The multiplier in the in-phase branch is connected directly to the local oscillator, whereas the multiplier in the quadrature branch is connected to a 90° phase-shifted version of the local oscillator. The information in the frequency-shift-keyed signal is then decoded and utilized according to its intended purpose.
Such an FSK demodulator is, for instance, described in U.S. Pat. No. 4,987,374, in the name of Burke. This demodulator comprises a local oscillator feeding a first and a second branch, each branch comprising a mixer and a detection stage. The mixer in the first branch mixes the incoming signal with the direct signal from the local oscillator, and the mixer in the second branch mixes the incoming signal with a 90° phase-shifted version of the signal from the local oscillator.
FSK receivers according to the prior art work satisfactorily in a multitude of applications. However, if the available power is only small, as is the case in hearing aids, the effective transmission range is very short, and reception errors, e.g. due to noise present in the signal, may severely corrupt the quality of the received signal.
More confident means of detecting the signals for the purpose of improving the noise-immunity of an FSK receiver without a significant increase in power consumption is thus desired.